The present invention relates to a temperature-compensated crystal oscillator for which temperature compensation is possible and which brings a signal of a predetermined frequency into oscillation, and to a method of performing the temperature compensation of the crystal oscillator.
Conventionally, the following four methods have been known as the methods of temperature compensating crystal oscillators.
(1) Direct Compensation Type
In this type, the oscillation loop of a crystal oscillator has a temperature compensating circuit comprising a capacitor and a resistor. During oscillating, each value of the capacitor and resistor is made to change depending on temperature, thereby realizing a stabilization of the oscillation frequency.
(2) Indirect Analog Compensation System
In this system, a crystal oscillator to be temperature-compensated is a voltage-controlled crystal oscillating circuit (VCXO) 3 as shown in FIG. 7. For temperature compensation, a temperature sensor 1 detects an operating temperature of the crystal oscillator, and a temperature compensation voltage generating circuit 2 generates a temperature compensation voltage for compensating the temperature characteristic of a crystal resonator 4 based on the above detected temperature. Then, by applying the temperature compensation voltage to a voltage control terminal of the voltage-controlled crystal oscillating circuit 3, the temperature compensation for it is performed.
When the crystal resonator is made of AT cut crystal, its temperature characteristic can be accurately approximated with a cubic function. Therefore, the temperature compensation circuit is generally composed of a temperature sensor having a temperature characteristic represented by a linear function, and a cubic function generating circuit that generates a cubic function depending on the temperature detected by the temperature sensor.
(3) Indirect Digital Compensation System
The basic concept regarding the temperature compensation of this system is similar to that of the indirect analog system described above, except that the temperature detected by the temperature sensor 1 is digitally processed to generate the temperature compensation voltage.
That is, in this system of temperature compensation, as shown in FIG. 8, the temperature detected by the temperature sensor 1 is AD converted into a digital value by an AD converter 5. Then, the temperature compensation data which is used to compensate the temperature characteristic of the crystal resonator 4, and which has been stored in advance at the address of a non-volatile memory 6 corresponding to the converted digital value is read. The temperature compensation data read is DA converted into the temperature compensation voltage in analog form by DA converter 7. By supplying the temperature compensation voltage in analog form to the voltage-controlled crystal oscillating circuit 3, the temperature compensation is performed. Here, an EEP-ROM or the like may be used as the non-volatile memory 6.
(4) Temperature Compensation by Constant Temperature Oven
In this method, a crystal oscillator is placed in a small constant temperature oven and thus kept at constant ambient temperature, thereby eliminating the temperature dependence of the crystal oscillator caused by variations in outside-air temperature. Therefore, the oscillation frequency is kept constant.
However, all of the conventional temperature compensation methods (1)-(4) have a problem that it is difficult to realize high-accuracy temperature compensation at low cost (frequency deviation in the temperature characteristic is less than about 0.01 ppm). The concrete contents of this problem are as follows.
Although the analog system of (1) and (2) can be realized at the lowest cost among the methods described above, it is difficult to realize a mass production level of the crystal oscillators with a medium degree of temperature compensation accuracy (below 1 ppm) due to the following reason. That is, it is difficult to improve the temperature compensation accuracy because, in the method of (1), variations in each of the passive elements associated with the temperature compensation have an effect on the accuracy, and, in the method of (2), the temperature compensating circuits are realized with analog circuits.
On the other hand, the method of (3) in principle has the possibility that it realizes the high-accuracy temperature compensation by increasing the resolution of the AD converters and DA converters and also increasing the storage capacity of the non-volatile memories. However, to this end, it is necessary that the bit count of the AD and DA converters is above 15 bits, and that, if the data is not interpolated, the storage capacity of the non-volatile memories is 15 bits (32768 addresses)xc3x9715 bits=about 500 kbits. Therefore, in integrating such circuits into an integrated circuit, it becomes a large-scale integrated circuit.
Furthermore, the method of (4) requires a small constant temperature oven and thus has a problem of considerably high cost.
Considering these problems, the inventor has conducted research with enthusiasm toward a solution to the problems. As a result, the inventor has found following matters; that is, first the temperature compensation of a crystal oscillator is performed by the technique of analog manner based on the temperature detected by a temperature sensor, and on the other hand, with respect to the error portion resulting from the temperature compensation the temperature compensation is performed by the technique of a digital manner based on the temperature detected by the temperature sensor, and consequently it is possible to realize decreased manufacturing cost by the smaller scale of the oscillator circuit and the high-accuracy temperature compensation.
An object of the invention attained by the new findings above described is to provide a temperature-compensated crystal oscillator and a method of temperature compensating a crystal oscillator which allow the realization of decreased manufacturing cost by a smaller scale circuit and the high-accuracy temperature compensation.
The invention provides a temperature-compensated crystal oscillator which includes a voltage-controlled crystal oscillating circuit controlled in oscillation frequency with a temperature compensation signal for performing the temperature compensation of a crystal resonator, and which brings the voltage-controlled crystal oscillating circuit into oscillation by the use of the crystal resonator. Further, the temperature-compensated crystal oscillator includes: a temperature sensor for detecting the operating temperature of the crystal oscillator; analog temperature compensating means for generating a temperature compensation voltage of an approximate quadratic function, an approximate cubic function, or an approximate quartic or more function corresponding to the temperature characteristic of the crystal resonator based on the temperature detected by the temperature sensor, and for supplying the generated voltage to the voltage-controlled crystal oscillating circuit; and digital temperature compensating means for AD converting the temperature detected by the temperature sensor, for outputting temperature compensation data in digital form which have been stored in advance in a memory in association with the AD converted value, for DA converting the temperature compensation data into the temperature compensation voltage in analog form, and for supplying the temperature compensation voltage to the voltage-controlled crystal oscillating circuit, and the temperature-compensated crystal oscillator is characterized in that the oscillation frequency of the voltage-controlled crystal oscillating circuit is controlled based on both of the temperature compensation voltages from both of said temperature compensating means.
Further, an embodiment of the temperature-compensated crystal oscillator of the invention includes a temperature-compensated crystal oscillator, characterized in that the analog temperature compensating means comprises cubic function generating means for generating a temperature compensation voltage of an approximate cubic function corresponding to the temperature characteristic of the crystal resonator based on the temperature detected by the temperature sensor, and a memory storing in advance data for setting a predetermined coefficient of the temperature compensation voltage generated by the cubic function generating means, wherein the coefficient generated by the cubic function generating means is set by the data read from the memory.
Furthermore, an embodiment of the temperature-compensated crystal oscillator of the invention includes a temperature-compensated crystal oscillator, characterized in that the digital temperature compensating means comprises an AD converter for AD converting the temperature detected by the temperature sensor, a non-volatile memory for generating the temperature compensation data in digital form which have been stored in advance in association with the AD converted value by the AD converter, and a DA converter for DA converting the temperature compensation data from the non-volatile memory into the temperature compensation data in analog form.
Still further, an embodiment of the temperature-compensated crystal oscillator of the invention includes a temperature-compensated crystal oscillator, characterized in that the analog temperature compensating means and the digital temperature compensating means are integrated into one chip on a semiconductor substrate.
According to the temperature-compensated crystal oscillator comprising these configuration of the invention, the analog temperature compensating means processes the temperature detected by the temperature sensor by the technique of an analog manner to generate a predetermined temperature compensation voltage corresponding to the detected temperature, and supplies the temperature compensation voltage to the voltage-controlled crystal oscillating circuit. On the other hand, the digital temperature compensating means processes the temperature detected by the temperature sensor by the technique of a digital manner to generate a predetermined temperature compensation voltage corresponding to the detected temperature, and supplies the temperature compensation voltage to the voltage-controlled crystal oscillating circuit.
For these reasons, according to the invention, the analog temperature compensating means can provide temperature compensation of medium or more accuracy, and the digital temperature compensating means can temperature compensate more finely for the error portion resulting from the temperature compensation of medium accuracy. Therefore, high-accuracy temperature compensation can be performed as a whole.
Further, according to the invention, as the analog temperature compensating means generates the temperature compensation voltage by the technique of an analog manner, the scale of the circuit for the analog temperature compensating means can be made relatively small. On the other hand, although the digital temperature compensating means generates the temperature compensation voltage by the technique of a digital manner, the bit count of the digital signals to be treated can be decreased and therefore the scale of the circuit for the digital temperature compensating means can be made relatively small. For these reasons, the scale of the whole circuit can be made smaller, resulting in the realization of decreased manufacturing cost.
Further, according to the temperature-compensated crystal oscillator of the invention, when the analog temperature compensating means and the digital temperature compensating means are integrated into one chip on a semiconductor substrate, the advantages described above can be realized easily and reliably.
The invention also provides a method of performing the temperature compensation of a crystal oscillator, characterized in that a method of performing the temperature compensation of a crystal oscillator by supplying a temperature compensation signal to a voltage-controlled crystal oscillating circuit comprises detecting the operating temperature of the crystal oscillator, generating a predetermined temperature compensation signal corresponding to the detected temperature by the technique of an analog manner, and thereby performing analog temperature compensation of the crystal oscillator based on the temperature compensation signal, and further comprises, on the other hand, for the error portion resulting from the analog temperature compensation, generating a predetermined temperature compensation signal corresponding to the detected temperature by the technique of a digital manner, and performing digital temperature compensation of the crystal oscillator based on the temperature compensation.
Further, an embodiment of the method of performing the temperature compensation of a crystal oscillator of the invention includes the temperature compensation method for a crystal oscillator, characterized in that the analog temperature compensation is based on the detected temperature and comprises a process for generating the temperature compensation voltage of an approximate cubic curve corresponding to the temperature characteristic of the crystal resonator.
Furthermore, an embodiment of the method of performing the temperature compensation of a crystal oscillator of the invention includes the temperature compensation method for a crystal oscillator characterized in that the digital temperature compensation comprises a first process of determining a AD converted value by AD converting the detected temperature, a second process of generating temperature compensation data in digital form which have been stored in advance in a memory in association with the AD converted value determined, a third process of DA converting the temperature compensation data into a temperature compensation voltage in analog form.
Thus, according to the invention, in performing the temperature compensation of crystal oscillator, the analog temperature compensation can perform temperature compensation with medium or more accuracy, and the digital temperature compensation can temperature compensate more finely for the error portion resulting from the analog temperature compensation, allowing the realization of the high-accuracy temperature compensation as a whole.
Further, according to the invention, as the analog temperature compensation generates the temperature compensation voltage by a technique of an analog manner, in realizing a circuit for the analog temperature compensation, the scale of the circuit can be made relatively small. On the other hand, although the digital temperature compensation generates the temperature compensation voltage by the technique of a digital manner, the bit count of digital signals to be treated can be decreased, and therefore, in realizing a circuit for the digital temperature compensation, the scale of the circuit can be made relatively small. For these reasons, in realizing the temperature-compensated crystal oscillator, the scale of the whole circuit can be made small, resulting in the realization of decreased manufacturing cost.